Forgive my ignorance on the subject. I'm fairly new to I&E design. Can someone enlighten me about where a 4 wire 4-20mA input module might be used besides an RTD?

Thanks

Mike

 

Any instrument that needs external power to power the transmitter (typical supply 110V/220VAC).

Instruments such as Magflow, Ultrasonic level transmitters, Density, analyzers etc.

 

Thanks. I was also wondering now if anyone has an example of a loop. My senior designer is telling me I need a 4 wire Delta V input card and I don't know why. This is a micromotion 1700 coriolis flow transmitter with MVD technology.

All of the other field powered loops I've done have only involved two-wire input or output cards.

Am I confused or is he?

 

It may be that the "4-wire" input card has the terminals for the loop power supply (integrated) and then terminals for the 4-20 ma. loop to the field devices. i.e. the micromotion 1700.

 

Yes, Von is correct. And here's another point to consider - with your other field-powered loops that involved two-wire modules, were the instruments set up as active/sourcing or passive outputs? They were probably set up as passive, as I believe most two-wire input modules (e.g. 4-20mA w/HART Input) are internally connected to the loop power supply so as to source the signal. One of the advantages to a 4-wire term block is that you can connect either active or passive instrument to the same input channel on the module, you just connect different terminals dedicated to that channel. Another advantage is, you can eliminate the need to add signal isolators into the loop. For instance, say you had an instrument that could onlyact as sourcing and you only had available two-wire input modules, which also sourced, then you would have "bucking" power supplies and would have to add an isolator in the loop. With a 4-wire module and terminal setup, you would only have to move a couple wires at the module.

FYI - Micro Motion also puts out a two-wire, loop-powered transmitter.